1. Field of the Invention
The present invention relates generally to a DC/DC converter and specifically to a system and method to accelerate commutation for a passive clamp isolated boost for a bi-directional DC/DC converter.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs.
Typically, a vehicle propelled by an electric motor can use batteries or fuel cells to generate the necessary current. Fuel cells generate electrical power through an electrochemical reaction of a fuel and oxidant, such as hydrogen and oxygen. Water is the product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen, a product that is easily disposed. See generally, U.S. Pat. No. 5,991,670 to Mufford.
The desirability of using electric motors to propel a vehicle is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. Nevertheless, new ways must be developed to optimize these potential benefits.
One such area of electric vehicle (EV) development is converting direct current (DC) generating devices such as fuel cells and high voltage (HV) batteries to their appropriate load. Ideally, the current generators (such as HV batteries or fuel cells) and loads (such as vehicle 12 V powered accessories) would all be at the same voltage level. Unfortunately, this is not presently the case. For example, a conventional 12 V voltage system is still needed in an electric vehicle to power conventional 12 V loads such as lights, sensors and controllers, while a high voltage bus (for example 300 V) feeds the traction inverter and motor. There is a dual-voltage power system in the electric vehicle and energy needs to be transferred bi-directionally between the two voltage systems.
Therefore, a successful implementation of electric traction motor propelled vehicles requires an effective bi-directional DC/DC converter. The converter must be bi-directional because the high voltage bus can be used as a current load during start-up or as a current generator. Similarly, the 12 V battery can be used as a current generator or as a load while charging. DC/DC converters are certainly known in the prior art. Even bi-directional DC/DC converters are known. See generally, U.S. Pat. No. 5,745,351 to Taurand and U.S. Pat. No. 3,986,097 to Woods.
In a bi-directional DC/DC converter, one side, the primary side, of the transformer can be current-fed and the other side, the secondary side, can be voltage-fed. It is well known in the prior art that the primary side normally experiences a high voltage overshoot during a diagonal switch-pair turn-off condition. This voltage spike needs to be clamped to avoid a voltage overshoot passing through the switching devices. A passive clamp converter employs a diode and a capacitor to absorb excessive energy from the voltage overshoot and a resistor to dissipate the absorbed energy. Unfortunately, the use of a simple prior art passive-clamped snubber results in severe limitation in a low voltage (12 V), high current (e.g., hundreds of amperes) application due to significant power loss, although it is a simple approach widely used by industry to resolve the voltage spike issue.
An active clamp in the prior art replaces the resistor in the passive clamp circuit with a switch to pump back the energy to the source when the capacitor is not absorbing energy. This recycles the dissipated energy and improves efficiency, but this technology is expensive to implement.
In the prior art, bi-directional flyback converters are known to be best suited for low power applications. Any automobile DC/DC converter must be able to withstand the extreme environmental conditions and higher power requirements experienced by many vehicles. Therefore, there is a desire and a need for an efficient and cost effective high power bi-directional DC/DC converter.
Accordingly, an object of the present invention is to provide an efficient and cost effective high power bi-directional DC/DC converter that can withstand the vigorous environment of an automobile.
The invention is a method and system to accelerate commutation for passive-clamped isolated high power boost converters comprising a primary circuit having at least one pair of diagonal controllers (such as switches or diodes); a secondary circuit comprising at least two controllers (such as switches or diodes); a one directional or bi-directional transformer connected to the primary circuit and the secondary circuit; and the primary circuit also having a snubber comprising a clamping diode, a clamping capacitor and an energy dissipater (such as a resistor or small buck converter). The clamping diode can include xe2x80x9cpush-pullxe2x80x9d and xe2x80x9cLxe2x80x9d configurations. The commutation of the present invention protects the primary circuit switches from voltage spikes in a boost mode.
The present invention shorts the secondary circuit with a method and system to turn on at least two switches in the secondary circuit simultaneously for a minimal calibratable period (for example, 2 microseconds) while primary circuit diagonal switches turn off, whereby the primary circuit current can be transferred to the secondary circuit quickly and reduce a voltage spike due to primary current and transformer leakage interaction.
The present invention also has a means to allow a smooth transition between a choke (inductor) current and a transformer primary current. Primary current increases linearly through the snubber circuit during switching conditions, thus protecting the primary circuit controllers.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.